1. Field of the Invention
This invention relates to improved metal halide lamps having oxidized metal frame parts or oxygen in the outer envelope. More particularly, this invention relates to metal halide arc discharge lamps having improved lumen maintenance wherein (i) the surface of the metal frame parts has been oxidized or (ii) the outer jacket contains oxygen or an oxidizing agent, or (iii) a combination thereof.
2. Background of the Disclosure
Metal halide lamps, as is well known to those skilled in the art, contain an electric light source comprising an arc tube made of a vitreous material such as quartz or a high temperature glass which is generally centrally supported within a vitreous outer envelope by metal parts referred to as frame parts. For the most part such frame parts are generally nickel plated steel which is inexpensive, corrosion resistant and which has a clear, shiny appearance that meets with customer acceptance. The outer envelope generally has a stem or neck-shaped portion on at least one end thereof which terminates in a substantially metal base portion. The arc tube or electric light source accommodated in the envelope is connected to the metal base by current supply conductors. The arc tube contains an electrode disposed at each end, mercury, a halide of sodium and a halide of one or more metals such as scandium, cesium, calcium, cadmium, barium, mercury, gallium, indium, thallium, germanium, tin, thorium, selenium, tellurium, etc. Generally the iodides of these metals are preferred, although the bromides and, in some cases chlorides may also be used. The arc tube also contains an inert gas such as argon. In the past it was fairly standard practice to support the arc tube within the vitreous outer envelope by attaching same to a metal harness. In this type of construction, the arc tube is supported within a harness by metal straps around the pinched ends of the arc tube with metal rods present along the sides of the arc tube so that a substantial amount of the metal harness structure is proximate to the arc tube. This type of harness support is disclosed, for example, in U.S. Pat. Nos. 3,452,238 and 3,569,766.
Some lamp manufacturers still employ such a harness frame type of mounting. However, J.F. Waymouth in Electric Discharoe Lamps, pp. 266-277, (MIT Press 1971) showed that photoelectrons ejected from the support rods of such frames, which carry lamp current, are responsible for the electrolysis of sodium from the arc tubes. This results in a depletion of the sodium level in the arc tubes and a concomitant increase in the required operating voltage of the lamp, along with a decrease in lumen output as the lamp ages and the sodium migrates from within the interior of the arc tube. His solution to this problem was to do away with the harness construction then used and replace it with a frame structure supporting the arc tube in which there are no structural support members present along the sides of the arc tube, and which utilizes a thin wire located away from the arc tube to carry the lamp current to one of the arc tube electrodes. However, he also found that this type of arc tube support structure provided little or no advantage if there was a vacuum in the outer jacket. Consequently, his solution employed a gas fill in the outer lamp jacket in combination with an arc tube support structure which is not proximate the sides of the arc tube. This type of lamp construction, is in wide use today and is shown, for example, in U.S. Pat. Nos. 3,937,996 and 4,581,557.